Without the data to try it out, I'm just going on a "hunch" here, but ...
I think when you use y*x=cd it will only connect the markers that have the same value of CD.
But you can "trick" it into looking like they are connected, by first doing a plot of just the line, and then doing a plot of the different markers, and overlay them. Something like the following (not 100% sure this code will work ver-batim, since I don't have the data)...
ps - I'm assuming you don't really want to plot "by cd date", because that would produce a separate plot for each cd and each date (that's not what you want, right?)
symbol1 interpol=none v=triangle c=red;
symbol2 interpol=none v=circle c=green;
symbol3 interpol=none v=square c=pink;
symbol4 interpol=none v=diamond c=purple;
symbol5 interpol=none v=dot c=tan;
symbol6 interpol=j v=none c=black;
proc gplot data=bill;
plot n*date=6 n*date=CD/ overlay haxis=axis2;
run;quit;